Individual animal condition connects forest structure with tick infestation and pathogen presence

Abstract

Forest structural complexity is widely assumed to enhance biodiversity, yet its effects on the physiological condition and health of individual organisms remain poorly understood. We examined how forest structural complexity relates to individual condition and, in turn, to ectoparasite and pathogen occurrence in free-living vertebrates. Across 19 temperate forest plots in Flanders (northern Belgium), spanning a gradient of structural complexity, we sampled birds and small mammals and quantified individual condition using scaled mass index, fluctuating asymmetry, and relative telomere length. Larval ticks were collected from hosts and screened for a broad panel of tick-borne pathogens. Relative telomere length tended to be lower in more structurally complex forests, whereas body condition and fluctuating asymmetry showed no consistent relationship with forest structure. Individuals in better body condition were less likely to carry larval ticks. In contrast, hosts with shorter telomeres were more likely to carry pathogen-infected larvae, consistent with links between cumulative physiological stress and infection susceptibility. No evidence was found for systematic co-occurrence among pathogens within larval ticks. Together, our results indicate that individuals interact with forest structure through multiple, potentially independent physiological pathways, shaping observed patterns of parasite load and tick-borne pathogen prevalence.

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